10 Questions
What material are glowers in IR spectrometers typically made of?
Silicon carbide
Which type of monochromator has become obsolete in IR spectrometers?
Prism
What type of detector is used in an IR spectrometer?
Thermocouple
Which of the following can be used as a sample in an IR spectrometer?
Pure liquid such as chloroform
What is the typical concentration range for a solid sample in a KBr disk in an IR spectrometer?
0.1-0.2%
Which absorption range is indicative of a C=C double bond?
near 1650 cm-1
What absorption region often implies the presence of an aromatic ring?
1600-1450 cm-1
A sharp absorption near 2250 cm-1 is commonly associated with which functional group?
CEN
Which absorption region is characteristic of a nitro group?
1600-1530 cm-1 and 1390-1300 cm-1
If none of the characteristic absorptions for double, triple bonds or nitro groups are found, which absorptions would you expect to find in a hydrocarbon?
3000 cm-1 and 1460-1375 cm-1
Study Notes
Functional Groups in IR Spectroscopy
Double bonds and/or aromatic rings
- C=C bonds absorb weakly near 1650 cm-1
- Medium-to-strong absorptions between 1600-1450 cm-1 often indicate aromatic rings
- Aromatic and vinyl C-H absorbances occur left of 3000 cm-1, distinguishing them from aliphatic C-H
Triple bonds
- C≡N bonds absorb medium, sharply near 2250 cm-1
- C≡C bonds absorb weakly, sharply near 2150 cm-1
- Acetylenic C-H absorbs near 3300 cm-1
Nitro groups
- Two strong absorptions between 1600-1530 cm-1 and 1390-1300 cm-1
Hydrocarbons
- No absorptions characteristic of other functional groups
- Major absorptions in C-H region near 3000 cm-1
- Simple spectrum with additional absorptions near 1460 and 1375 cm-1
IR Spectrometer Components
Light Source
- Globar (electrically heated silicon carbide rod)
- Nernst glower (small heated refractory oxide rod)
Monochromators
- Prism (obsolete)
- Grating (better resolution, especially at high frequency)
Detector
- Thermocouple detector
IR Spectroscopy Process
- Light passes through a sample, and frequencies matching vibrational frequencies are absorbed
- Remaining light is transmitted to a detector
Sample Preparation
- Pure liquid samples (e.g., acetone, chloroform, methanol)
- Solid in liquid (dissolve solute in solvent)
- Solid sample in KCl or KBr disk (transparent disk, 0.1-0.2% concentration, e.g., 2 mg sample in 200 mg KBr)
Learn to identify double bonds, aromatic rings, and triple bonds in infrared spectroscopy through characteristic absorption patterns. Understand how to confirm these bonds by analyzing the C-H region. A must-know for organic chemistry students!
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